Golang: append file to an existing tar archive

The tar file specification states:

A tar archive consists of a series of 512-byte records. Each file system object requires a header record which stores basic metadata (pathname, owner, permissions, etc.) and zero or more records containing any file data. The end of the archive is indicated by two records consisting entirely of zero bytes.

The Go implementation of adding these two zero filled records happens here .

To get around the tar file format trailer (basically 1024 bytes of nothing) you could replace the lines:

f, err = os.OpenFile("/home/jeff/Desktop/test.tar", os.O_APPEND|os.O_WRONLY, os.ModePerm)
if err != nil {
    log.Fatalln(err)
}
tw = tar.NewWriter(f)

With:

f, err = os.OpenFile("/home/jeff/Desktop/test.tar", os.O_RDWR, os.ModePerm)
if err != nil {
    log.Fatalln(err)
}
if _, err = f.Seek(-1024, os.SEEK_END); err != nil {
    log.Fatalln(err)
}
tw = tar.NewWriter(f)

It opens the file read / write (instead of append / write-only) and then seeks to 1024 bytes before the end of the file and writes from there.

It works, but it is a horrible hack.

EDIT: After understanding the tar file spec a little better, I no longer believe this is such a hack.

Full code: http://play.golang.org/p/0zRScmY4AC

It's just a writer interface so write bytes to it after writing your files header.

import (
  "archive/tar"
  "os"
)

f, err := os.OpenFile(path, os.O_APPEND|os.O_WRONLY, os.ModePerm)
if err != nil {
// handle error here
}

hdr := tar.Header{}
// populate your header
tw := tar.NewWriter(f)
// append a file
tw.WriteHeader(hdr)
tw.Write(content_of_file_as_bytes)

http://golang.org/pkg/archive/tar/#Writer tells you all you need to know.

EDIT: It turns out that tar files get a trailer written to the end when it's closed. So even though you are writing new data to the tar archive it won't be read past that trailer. So instead it looks like you'll have to read in the tar archive first and then rewrite the whole archive to disk which is suboptimal. The package doesn't support the necessary stuff to append to them though so that's the best I can recommend right now.